Method and apparatus for selecting trees for harvest

ABSTRACT

A method for selecting trees for harvest according to a predetermined criterion is provided which includes at least the steps of applying a vibrative member to the tree, vibrating the vibrative member, determining the resonance properties of the vibrative member, calculating an observed quality factor associated with the vibrative member vibrations, and, comparing the observed quality factor with a predetermined relationship between the quality factor and the tree selection criterion. A portable tree probe, suitable for field use, is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/416,062, filed Oct. 4, 2002, the contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus fortesting trees. More particularly, the present invention is directed to amethod and apparatus for selecting trees for harvest by testing thematerial properties of the trees in the field.

2. Description of the Related Art

It is known that it is necessary for lumber companies to test trees inorder to determine which ones are to be cut down. For example, maturetrees are selected for harvesting, while immature trees are leftstanding for further growth. One way to accomplish this is by taking acore sample of the tree and sending it to a laboratory to determine thedensity of the wood, which is related to the maturity of the tree.However, this process in undesirable because it is slow and injures thetree as coring typically produces a hole of from 1 to 3 inches indiameter which extends to the center of the tree. An opening of thissize in the tree left by the coring process can provide an entryway intothe interior of the tree for tree pathogens such as bacteria, fungi, andinsect pests that ultimately cause the tree to rot.

Various methods for testing the material properties of trees in thefield are known. For example, U.S. Pat. No. 3,877,294 to Shaw disclosesa vibration technique for rot detection in wooden poles and trees. Thetechnique includes applying a mechanical vibrational force at sonicfrequencies to, for example, a pole to be tested for rot, and measuringthe level of energy emerging from a number of axially spaced pointsalong the length of the pole and comparing the measurements of theemergent energy at the respective points. Shaw teaches that decayinduced rot manifests itself as a material of lower density than goodquality wood. The less dense material presents a lower impedance and,for a given resonant-like input signal, emergent energy is higher thanthat passing through good wood.

What is needed, however, is a simple and inexpensive method fordetermining tree maturity in the field.

SUMMARY OF THE INVENTION

A method for selecting trees in accordance with a predeterminedcriterion is provided herein. In one embodiment of the presentinvention, a method for selecting trees is provided comprising applyinga vibrative member to the tree, the vibrative member being characterizedby mechanical vibration resonance properties; vibrating the vibrativemember; determining the resonance properties of the vibrative member;calculating an observed quality factor associated with the vibrativemember vibrations; and, comparing the observed quality factor with apredetermined relationship between the quality factor and the treeselection criterion.

Another embodiment of the present invention is a portable tree probe,suitable for field use, comprising (a) a vibrative member having awood-penetrating end portion with at least one mechanical resonancefrequency; (b) means for vibrating the vibrative member at about theresonance frequency of the wood-penetrating end portion; and, (c) meansfor measuring vibration amplitude across a frequency range sufficient todetermine a characteristic Q value.

The method and portable tree probe of the present inventionadvantageously provide a way for trees to be tested in order todetermine if a tree should be harvested, based on its maturity, withminimal damage to the tree. Thus, trees that are not cut down can remainstanding for further growth and maturity without suffering from suchproblems as rotting.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are described below with reference to the drawingswherein:

FIG. 1 is a schematic diagram of an embodiment of the apparatus of theinvention applied to a tree;

FIG. 2 is an alternative embodiment of the apparatus of the invention;and,

FIG. 3 is a graph of resonance patterns.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The method of the present invention employs a probe having apredetermined resonant frequency of mechanical vibration. Resonances canbe characterized by a quality factor, or Q. A simple definition of the Qfactor is that it is the ratio of the resonance frequency f₀ to thefrequency 3 dB bandwidth f, or, as set forth in equation I:

Q=f ₀/(f ₂ −f ₁)  (I)

wherein f₁ and f₂ are the half power points.

In general, the resonance of a tuning fork depends upon such factors as,for example, the size, shape and materials of its manufacture. When notattached to a tree, the tuning fork has a well defined resonantfrequency profile with a high value (typically over 100) for the Qfactor. However, when embedded into a tree the Q value will drop becauseof the damping forces exerted by the tree. These damping forces arerelated to the viscoelastic properties of the tree, which, in turn, arerelated to the tree maturity. Accordingly, by measuring the Q value ofthe embedded tuning fork in a target tree and comparing that value withpre-measured Q values associated with trees of the same type and ofpredetermined ages, the maturity of the target tree, and hence thesuitability for harvesting the tree, can be determined. It should beunderstood that the viscoelastic property of the tree, and hence themeasurement of the Q value, is independent of the size and shape of thetree, as well as the surrounding soil type and conditions.

Referring now to FIG. 1, an example tree probe apparatus 100 of thepresent invention includes a unitary, monolithic vibrative member 101having a shaft 105 with a distal wood-penetrating end portion 102 and ahead portion 103 at the proximal end of shaft 105. The distalwood-penetrating end portion 102 includes at least one prongcharacterized by a resonance frequency of mechanical vibration. The treeprobe apparatus 100 is shown embedded in tree 10. A piezoelectric firsttransducer 110 is attached to the vibrative member 101 to cause it tovibrate at about the resonant frequency of the wood-penetrating endportion 102. An accelerometer 120 detects the amplitude of thevibrations of the tree probe at selected frequencies.

More particularly, the tree probe longitudinal vibrative member 101 isfabricated from a metal such as, for example, steel (e.g., carbon steel,stainless steel or other steel alloys), aluminum, non-ferrous alloys orany other suitable material (e.g., ceramic, plastic, etc.). Thewood-penetrating end portion can have one or more prongs, eachpreferably having a tapered end terminating in a sharp point tofacilitate the penetration of wood. In the event that a single prong isemployed (as shown in FIG. 2) the prong can be threaded to facilitatescrewing the tree probe into the wood of the tree. As shown in FIG. 1,the tree probe 100 can include two spaced apart parallel prongs 102 aand 102 b at the distal wood-penetrating end. The prongs 102 a and 102 bare preferably constructed so as to have different resonancefrequencies. A head-portion 103 has a proximal facing stop surface 103 awhich can be hammered to drive the tree probe 100 into the tree.

First transducer 110 is attached to the vibrative member 101 and appliesvibrational energy to the tree probe 100 at or close to the resonantfrequency of the prong(s) of the tree probe 100. Generally, thetransducer 110 is piezoelectric in operation and converts electricalenergy to mechanical energy. Piezoelectric stacks for use as transducer110 are known. Suitable piezoelectric materials include, but are notlimited to, quartz and piezoelectric ceramics such as, for example,barium titanate. Quartz has a high Q value but is more brittle. Bariumtitanate has high resistance to mechanical vibration and shock, as wellas good chemical resistance. The size of transducer 110 can vary widely,e.g., the size can be relatively small, i.e., less than about 10 mm on aside and less than about 1 mm thick, and can be in the form of variousshapes, e.g., rectangular, cylindrical, or annular shapes. Transducer100 can be tunably driven by a variable frequency square wave or sinewave alternating current generator 111 or a resonant circuit preset tothe resonant frequency of the one or more prongs 102 a/102 b. Suchcircuits are known in the art. Various transducers are known in the artand are commercially available. One such transducer suitable for use inthe present invention as the first transducer is available under thedesignation/model number AE0203 D04 from Thor Labs.

An accelerometer 120 is attached to the vibrative member 101 of the treeprobe to detect the vibrations and produce an electric signalproportional to the amplitude of the vibrations, which is analyzed by aQ meter 121 or is otherwise frequency scanned to provide amplitude dataacross a frequency spectrum which includes the resonant frequency of theprong. For example, by plotting the amplitude of the vibrations across afrequency spectrum one can obtain a characteristic curve from which theresonance bandwidth can be measured. The Q value can be determined bycalculating the ratio between the resonant frequency and the bandwidth.Once the Q value is obtained for a particular tree it can be comparedwith Q values known for trees of various ages. For example, trees ofknown ages (determined, for example, by core sampling) can be tested todetermine their Q values. A relationship between Q value and tree agefor a type of tree (e.g., pine, oak, maple, etc.) can then be determinedand recorded in the form of a chart, graph, or empirically derivedmathematical formula. The Q value of a target tree of the same type canthen be used to determine the age of the tree in accordance with thepredetermined relationship.

The apparatus described herein is adapted for portability and use in thefield. Generally, the tree probe 100 can have an overall length Lranging from about 75 mm to about 200 mm, and a diameter ranging fromabout 3 mm to about 15 mm, and preferably from about 5 mm to about 10mm. The length L₁ of the prong(s) can typically range from about 5 mm toabout 50 mm. The length L₂ of the shaft 105 typically can range fromabout 15 mm to about 150 mm. The shaft 105 has its own resonance and thedimensions of the shaft should be selected such that the shaft resonancedoes not overlap the resonance of the prong(s). The distance ofpenetration L₃ into the tree is limited to the distance between thesharp points of prongs 102 a and 102 b the position of the transducer110 or accelerometer 120, which can typically range from about 5 mm toabout 100 mm. These ranges are given for purpose of exemplification, anddistances outside of these ranges can be used when appropriate. The treeprobe 100 is inserted into a tree, e.g., by hammering the woodpenetrating end of the vibrative member into the trunk of the tree. Thetransducers 110 and 120 are then attached to the side of vibrativemember 101 of the tree probe and vibration of the tree probe is inducedthrough transducer 110. The vibrations are then picked up byaccelerometer 120 and the Q value is calculated as mentioned above. Ifthe tree meets the appropriate criterion for harvest it can be cut forlumber. If it does not, the tree can be left to remain standing. In thismanner, the tree can further grow and mature with minimal damage causedby the testing procedure and apparatus.

Referring now to FIG. 2 another embodiment 200 of the tree probe havingonly a single prong is illustrated. Tree probe 200 includes a unitarymonolithic vibrative body member 201 having a cylindrical shaft 205 anda wood-penetrating end portion 204 extending distally from the shaft205. A prong 202 of the wood penetrating end portion 204 includes asharp distal point 202 a. A head 203 has a proximal facing stop surface203 a which can be hammered to drive the prong 202 into the wood of thetree 10. Alternatively, the prong 202 can be threaded and head 203 canhave a multisided (e.g. hexagonal) periphery to facilitate grasping by awrench. A piezoelectric stack 210 is attached to a wall surface in arecess 206 in the shaft 205. An accelerometer 220 is attached to anotherwall surface in the recess 206 of the shaft 205. The piezoelectric stack210 is electrically attached by wire(s) 211 to a means (not shown) forelectrical excitation at a predetermined frequency. The accelerometer220 is electrically attached by wire(s) 221 to means for converting theelectrical signal of the accelerometer into data for calculating aquality factor.

Various features of the invention are shown by the non-limiting exampleset forth below.

EXAMPLE

This example presents calculations illustrating the effects of dampingon a tree probe having a single prong such as that depicted in FIG. 2.The fundamental resonance frequency f_(n) of the prong depends upon thespring constant k and the mass of material M. For a cantilevered rod thefundamental frequency is estimated by the following formula II:$\begin{matrix}{{f_{n} = {{\frac{n}{2\pi}\sqrt{\frac{k}{M}}} = {\frac{n}{2\pi}\sqrt{\frac{3\pi \quad r^{4}E}{4{L^{3}\left( {m_{c} + {0.24\quad \frac{\pi \quad r^{2}L}{\rho}}} \right)}}}}}},} & ({II})\end{matrix}$

wherein E is the elastic modulus, m^(c) is the mass of the tip, r is theradius of the rod, L is the length of the rod and Δ is the density ofthe material

The proposed tree probe for this example is fabricated from steel havingan elastic modulus of 2.1×10¹¹ N/m² and a density of 7.65 g/cm³. Thesingle prong (e.g., prong 202) is a cantilevered rod, having a diameterof 3.175 mm and a length of 15 mm. Using the dimensions and values givenabove the fundamental frequency of the proposed tree probe is calculatedto be 10.4 kHz.

The damping effect of the tree fibers on the vibration amplitude of animplanted tree probe as a function of frequency f can be illustrated bythe following formula III: $\begin{matrix}{{X\left( {f,\zeta} \right)} = {\frac{1}{\sqrt{\left( {1 - \frac{f^{2}}{f_{n}^{2}}} \right)^{2} + \left( \frac{2\zeta \quad f}{f_{n}} \right)^{2}}}.}} & ({III})\end{matrix}$

wherein X_((f,ζ)) is the amplitude at frequency f, ζ is the dampingcoefficient, and f_(n) is the calculated resonant frequency of theprong, e.g., 10.4 kHz.

Referring now to FIG. 3, a graph illustrates calculated vibrationamplitudes for the tree probe for three damping coefficients across afrequency spectrum. The amplitudes are in arbitrary units. Plot Adepicts the amplitude variation for a damping coefficient of 0.01. Theamplitude at resonance is 50 units. The corresponding Q value for plot Ais calculated to be 29. Plot B depicts the amplitude variation for adamping coefficient of 0.02. The amplitude at resonance is 25 units. Thecorresponding Q value for plot B is calculated to be 14.6. Plot Cdepicts the amplitude variation for a damping coefficient of 0.03. Theamplitude at resonance is 16.67 units. The corresponding Q value forplot C is calculated to be 9.54.

This Example shows how Q values relate to damping effects upon aresonant tree probe. Determination of the Q value can provide anindication of the tree maturity wherein the damping effects on the treeprobe vibrations caused by the viscoelastic properties of the tree woodvary with the age of the tree.

While the above description contains many specifics, these specificsshould not be construed as limitations of the invention, but merely asexemplifications of preferred embodiments thereof. Those skilled in theart will envision many other embodiments within the scope and spirit ofthe invention as defined by the claims appended hereto.

What is claimed is:
 1. A method for selecting a tree according to apredetermined criterion, comprising the steps of: a) embedding avibrative member in the tree, the embedded vibrative member havingmechanical vibration resonance properties, including a resonancefrequency and a resonance bandwidth; b) mechanically vibrating thevibrative member at or near the vibrative member resonance frequency; c)determining the resonance properties of the vibrative member, includingthe vibrative member resonance frequency and bandwidth; d) calculatingan observed quality factor of the vibrative member based on thedetermined vibrative member resonance frequency and bandwidth; and, e)comparing the observed quality factor with a predetermined relationshipbetween the quality factor and the tree selection criterion.
 2. Themethod of claim 1 wherein the vibrative member has a wood-penetratingend portion characterized by the resonance frequency of mechanicalvibration.
 3. The method of claim 2 wherein the step of embedding thevibrative member to the tree comprises embedding the wood-penetratingend portion of the vibrative member into a trunk portion of the tree. 4.The method of claim 2 wherein the vibrative member is fabricated from ametal selected from the group consisting of stainless steel, steelalloys, aluminum and non-ferrous alloys.
 5. The method of claim 2wherein the vibrative member is fabricated from a material selected fromthe group consisting of ceramic and plastic.
 6. The method of claim 1wherein the tree selection criterion is dependent upon the maturity ofthe tree.
 7. The method of claim 2 wherein the wood-penetrating endportion includes at least one prong.
 8. The method of claim 2 whereinthe wood penetrating end portion includes two prongs.
 9. The method ofclaim 8 wherein the two prongs are each characterized by a differentresonance frequency.
 10. A tree probe comprising: a) a vibrative memberhaving a wood-penetrating end portion characterized by at least oneresonance frequency of mechanical vibration; b) means for mechanicallyvibrating the vibrative member, when the end portion thereof is embeddedin a tree, at about the resonance frequency of the wood-penetration endportion; and, c) means for measuring vibration amplitude of the embeddedvibrative member across a frequency range sufficient to include (i) theat least one resonance frequency, and (ii) a resonance bandwith of theembedded vibrative member, so as to determine a characteristic Q valuethereof.
 11. The tree probe of claim 10 wherein the wood penetrating endportion includes at least one prong.
 12. The tree probe of claim 10wherein the wood penetrating end portion includes two prongs.
 13. Thetree probe of claim 12 wherein the two prongs are each characterized bya different resonance frequency.
 14. The tree probe of claim 10 whereinthe vibrative member is fabricated from a metal selected from the groupconsisting of stainless steel, steel alloys, aluminum and non-ferrousalloys.
 15. The method of claim 10 wherein the vibrative member isfabricated from a material selected from the group consisting of ceramicand plastic.
 16. The tree probe of claim 14 wherein the vibrative memberis a unitary single piece member.
 17. The tree probe of claim 15 whereinthe vibrative member is a unitary single piece member.
 18. The treeprobe of claim 16 wherein the means for vibrating the vibrative membercomprises a piezoelectric transducer attached to the vibrative memberand means for supplying the piezoelectric transducer with an alternatingcurrent at about the resonance frequency of the wood penetration endportion of the vibrative member.
 19. The tree probe of claim 18 whereinthe means for supplying an alternating current includes a tunable sinewave or square wave generator.
 20. The tree probe of claim 10 whereinthe means for measuring vibration amplitude includes an accelerometerattached to the vibrative member.
 21. The tree probe of claim 10 whereinthe means for vibrating the vibrative member comprises a piezoelectrictransducer attached to the vibrative member and means for supplying thepiezoelectric transducer with an alternating current at about theresonance frequency of the wood penetration end portion of the vibrativemember and wherein the means for measuring vibration amplitude includesan accelerometer attached to the vibrative member.